1. Field of the Invention
This invention relates to a demodulator and more particularly, an improved quadraphase shift keyed adaptive demodulator.
2. Prior Art
To meet the increasing demand for ultra high speed data transmission, in recent years, quadraphase shift keyed transmission systems are commonly used to take advantage of its capability for handling higher rates of data transmission. In quadraphase shift keyed data transmission systems, usually several different techniques have been utilized to demodulate the quadraphase shift keyed signal. Of the several techniques, the so-called Costas demodulation is one of the more popular approaches. The Costas demodulation approach entails use of an error signal derived from the inphase and quadraphase channels as follows. The output of the tracking VCO (voltage control oscillator) of the demodulator is split into quadrature components by a 90.degree. hydrid phase splitter. The quadrature components are then supplied to the two phase detectors for the I and Q channels, respectively. The respective phase detector of the two channels demodulates the incoming digital signals. The demodulation also entails cross multiplication of the I and Q channels to obtain the cross talk between channels. The measure of the cross talk results in an error signal that drives the reference VCO. When the channel cross talk is minimized, the frequency and phase of the reference VCO are assumed to be correct for proper coherent demodulation.
The Costas approach suffers from a number of shortcomings one of which one is its inability to handle the modulator and demodulator phasor errors in a satisfactory manner and the other is that the conventional Costas approach requires the phase locking of the demodulator.